Fireriver uses Live Bookmarks to build a River of News in your browser. No unread counts, no 3-pane view like an email client. Just paddle down the page with the space bar or your scroll wheel.

Organize the Live Bookmarks into folders to split things up into multiple rivers—which is not a strict River of News, per se. But, I like using them to focus on particular topics or priorities I happen to have time for.

It also displays a more obvious notification when a site has a feed and lets you add a new Live Bookmark subscription to the Fireriver folder with one click. (I'm considering splitting this feature off into its own small add-on.)

Of course, when I say experimental, I mean that this is my first Firefox add-on ever. I'm starting to use it daily, but I'm well-versed in nuking and rebuilding my Firefox profile. I haven't had to do that yet, but I'm expecting to have to do it eventually. I'll be pleasantly surprised if it turns out I haven't totally screwed something up.

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OPML Support - I just used this to dump in 800 subscriptions from Google Reader

There is a bit of a sync story, though. If you're using Firefox Sync, the Live Bookmark subscriptions will carry across machines along with the rest of your bookmarks. Also, I'm using history to hide items from display, and that is covered by sync too.

I need to check, but I don't think items within the Live Bookmarks are sync'd, so each machine is on its own for doing the actual feed polling.